Sequences homologous to the Moloney-murine sarcoma virus (MSV) transforming gene (v-mos) were found to be present within a unique 2.5 kbp Eco RI fragment of human DNA. This fragment (designated c-mos(human)) was molecularly cloned from human placental DNA using a bacteriophage vector. Comparison of v-mos and c-mos(human) by restriction enzyme and heteroduplex mapping techniques revealed that c-mos(human) possesses a stretch of 650 nucleotides of continuous homology related to v-mos (1100 nucleotides). Attempts to demonstrate biological activity of c-mos(human) have thus far been unsuccessful. C-mos(human) was used as a probe to localize mos sequences within the human genome to chromosome 8.